Building façades are essential to urban aesthetics and functionality, yet they face persistent degradation from weathering, biological colonization, and particulate deposition. Beyond their architectural role, façades play a key role in hygrothermal regulation and indoor comfort. Addressing these challenges, photocatalysis offers a promising solution, leveraging light-driven redox reactions to degrade pollutants while imparting surfaces with self-cleaning, biocidal, and air-purifying capabilities. These processes often induce superhydrophilicity, enhancing resistance to fouling. However, conventional photocatalytic systems often rely on costly, non-renewable materials. This study proposes a sustainable alternative: a marine-derived photocatalyst based on calcined oyster shells (biogenic lime), functionalized with titanium dioxide (anatase TiO₂) and silver (Ag) via incipient wetness impregnation, followed by calcination (at 450 °C with a 2-h ramp-up and a 2-h hold), benchmarked against pure TiO₂ as a reference. The marine-based material was applied to lime mortar coated with silicate paint and characterized via SEM-EDS. The photocatalytic performance was assessed through rhodamine B degradation under UV light and water interaction tests, including contact angle measurements and microdrop absorption time. The marine-based photocatalyst achieved 30–40% of the self-cleaning efficiency of commercial TiO₂ when normalized considering the color change observed for the first, despite having roughly half the photoactive atomic concentration, indicating potential for future optimization.